Method and system for facilitating burst-mode optical power maeasurement

ABSTRACT

An optical line termination comprises a DC current load, power measurement circuitry, upstream data path circuitry and current mirror circuitry. The current mirror circuitry is connected between the DC current load, the power measurement circuitry and the upstream data path circuitry. The DC current load is connected in parallel with a photodiode of the upstream data path circuitry. The DC current load exhibits a substantially fixed load. The current mirror provides a copy of an aggregate current to the power measurement circuitry. The aggregate current is a summation of a current draw by the DC current load and a current draw by the photodiode. The power measurement circuitry is configured for outputting a power level dependent upon the aggregate current. Accordingly, the optical line termination provides for a non-intrusive solution for measuring optical input power and, thereby, enables the measured optical power to be monitored.

FIELD OF THE DISCLOSURE

The disclosures made herein relate generally to optical powermeasurement in a passive optical network and, more particularly, totechniques for facilitating burst-mode optical power measurement.

BACKGROUND

Conventional optical input power measurement solutions for optical datanetworks consist of average current-to-voltage conversion performed by atrans-impedance amplifier (TIA) or limiting amplifier (LimAmp). The TIAor LimAmp is provided in a receive data path of the optical datanetwork. In the case of a Passive Optical network (PON), a primaryfunction of use of one of these devices in an Optical Line Termination(OLT) application is to convert high-frequency photodiode current todigital voltage levels for data recovery. In some cases the TIA orLimAmp also provides an output voltage proportional to average opticalinput power, which is used by an external circuit to generate a ReceivedSignal Strength Indication (RSSI) measurement.

Known optical network technologies such as BPON (i.e., Broadband PassiveOptical Network) and GPON (i.e., Gigabit Passive Optical Network) useburst-mode transmission at relatively high bit rates. Accordingly, BPONand GPON are referred to herein as burst-mode enabled PON technologies.Facilitating input power measurement for burst-mode enabled PONtechnologies requires implementing functionality in a network's MediaAccess Controller (MAC) to coordinate Optical Network Unit (ONU) burstwith the RSSI measurement. However, to date, no solutions for MAC-layerfunctions required for facilitating such conventional optical inputpower measurements are known to exist. One example of such MAC-layerfunctionality is facilitating control of an Analog-to-Digital Converter(ADC) for converting analog signalling information to correspondingdigital signalling information.

Without imposing significant added cost and complexity, conventionaloptical input power measurement solutions will exhibit one or morelimitations when implemented in relatively high bit rate applicationssuch as GPON. One limitation is the response time for facilitating suchmeasurements when using conventional optical input power measurementsolutions. It is typically in excess of 1 millisecond. In a burst-modesystem such as a GPON, where a timeslot on the shared medium (typicallymicroseconds) must be allocated for the measurement, the time taken tomeasure power from one ONU affects the ability of the system to meetquality-of-service requirements for other end users. Accordingly, in aGPON system, a response time on the order of milliseconds is consideredunacceptable. Another limitation is the dynamic range associated withsuch measurements. It is limited by the use of a voltage output thatvaries linearly with average photodiode current, which results involtage output on the order of volts at the high end of the input powerrange and millivolts at the low end of the input power range. In thecase where this output voltage is referenced to ground potential, greatcare is required in the circuit implementation to overcome effects suchas noise and offset voltages. An ADC utilized in facilitating measure ofthe input voltage is required to have relatively high resolution inorder to meet the accuracy requirement at low power levels. Thisresolution requirement contributes to increased cost as well as longerconversion time. Additionally, TIA transimpedance, which determines thecurrent-to-voltage gain and therefore affects accuracy, tends to varysignificantly over temperature and device lot. Still another limitationis that accuracy is less than acceptable. Conventional optical inputpower measurement solutions that utilize linear current-to-voltageconversion require calculation of a logarithmic function to convertvoltage to optical power in units of dBm. Such a calculation or tablelook-up on an OLT will contribute additional error due to limitedprocessing power and/or memory of the OLT. One further limitation isthat RSSI measurement solutions that are built into a TIA or other datapath devices have the effect of limiting which devices can be used inthe data path.

Therefore, facilitating optical input power measurement in a manner thatat least partially overcomes limitations associated with conventionalapproaches for facilitating optical input power measurement would beuseful and advantageous.

SUMMARY OF THE DISCLOSURE

The present invention provides for a non-intrusive solution formeasuring optical input power and, thereby, enables the measured opticalpower to be monitored. In doing so, the present invention enablesoptical system equipment to adjust system parameters during normaloperation in order to improve system performance dependent upon themeasured optical input power. For example, optical network technologiesthat utilize burst-mode transmission at relatively high bit rates (e.g.,GPON and BPON) are particularly well-suited for and benefit fromimplementing such a non-intrusive measuring solution because informationderived from the optical input power measurements can be used to reducethe performance requirements of optical interfaces of an associatedpassive optical network, thus supporting improvements in systemperformance. Additionally, facilitating optical input power measurementsin accordance with the present invention offers faster response, greaterdynamic range and greater accuracy than that offered by conventionalmeasurement solutions, which is mandatory for adjusting systemparameters dependent upon optical input power measurements insophisticated, high-bandwidth optical network technologies such as GPONand BPON.

In one embodiment of the present invention, an optical line terminationcomprises a DC current load, power measurement circuitry, upstream datapath circuitry and current mirror circuitry. The current mirrorcircuitry is connected between the DC current load, the powermeasurement circuitry and the upstream data path circuitry.

In another embodiment of the present invention, an optical input powermonitoring apparatus comprises current mirror circuitry and powermeasurement circuitry. The current mirror circuitry includes a firstoutput and a second output. The power measurement circuitry is connectedto the second output. The current mirror circuitry is configured forproducing a copy of a first current, for outputting the copy of thefirst current through the second output and for outputting the firstcurrent through the first output.

In another embodiment of the present invention, a method is configuredfor generating a received signal strength indication. An operation isperformed for providing a copy of an aggregate current. The aggregatecurrent is a summation of a current draw by a DC current load and acurrent draw by a photodiode of upstream data path circuitry in apassive optical network. After providing the copy of the aggregatecurrent, an operation is performed for converting the aggregate currentto a digital voltage, followed by an operation being performed forconverting the digital voltage to a power level.

Turning now to specific aspects of the present invention, in at leastone embodiment, the DC current load is connected in parallel with aphotodiode of the upstream data path circuitry.

In at least one embodiment of the present invention, the DC current loadexhibits a substantially fixed load.

In at least one embodiment of the present invention, the current mirrorprovides a copy of an aggregate current to the power measurementcircuitry, and the aggregate current is a summation of a current draw bythe DC current load and a current draw by the photodiode.

In at least one embodiment of the present invention, the powermeasurement circuitry includes analog amplification circuitry andanalog-to-digital converting circuitry, the analog amplificationcircuitry is connected between the current mirror circuitry and theanalog-to-digital converting circuitry, the current mirror circuitryprovides a copy of an aggregate current to the analog amplificationcircuitry, and the aggregate current is a summation of a current draw bythe DC current load and a current draw by a photodiode of the upstreamdata path circuitry.

In at least one embodiment of the present invention, the powermeasurement circuitry includes a first analog filter connected betweenthe current mirror circuitry and the analog amplification circuitry, anda second analog filter connected between the analog amplificationcircuitry and the analog-to-digital converting circuitry.

In at least one embodiment of the present invention, the analogamplification circuitry outputs an analog voltage dependent upon theaggregate current; the analog-to-digital converting circuitry outputs adigital voltage dependent upon the analog voltage, and the powermeasurement circuitry includes a power measurement module that outputs apower level dependent upon the digital voltage.

In at least one embodiment of the present invention, the powermeasurement circuitry include a counter and the power measurementcircuitry is configured for starting the counter in response to anexpected change in at least one of the analog current and the analogvoltage and for initiating an analog-to-digital conversion of the analogvoltage when the counter attains a prescribed number of counts.

In at least one embodiment of the present invention, an operation forproviding a copy of the aggregate current includes providing to currentmirror circuitry both a current draw by a DC current load and a currentdraw by a photodiode of upstream data path circuitry.

In at least one embodiment of the present invention, converting theaggregate current to the digital voltage includes providing theaggregate current to analog amplification circuitry for generating ananalog voltage dependent upon the aggregate voltage and providing theanalog voltage to analog-to-digital converting circuitry for generatinga digital voltage dependent upon the analog voltage.

In at least one embodiment of the present invention, converting thedigital voltage to the power level includes providing the digitalvoltage to a power measurement module for generating a power leveldependent upon the digital voltage.

In at least one embodiment of the present invention, converting theanalog voltage to the digital voltage include starting a counter inresponse to energizing the photodiode and initiating ananalog-to-digital conversion of the analog voltage when the counterattains a prescribed number of counts.

In at least one embodiment of the present invention, the converting thedigital voltage to the power level is performed in response tocompletion of the converting the aggregate voltage to the digitalvoltage.

These and other objects, embodiments advantages and/or distinctions ofthe present invention will become readily apparent upon further reviewof the following specification, associated drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of an optical line termination includingreceived signal strength indication circuitry in accordance with thepresent invention.

FIG. 2 depicts an embodiment of a GPON OLT configured for providingclosed-loop photodiode gain control in accordance with the presentinvention.

FIG. 3 depicts an embodiment of a control loop used in the OLT in FIG. 2

DETAILED DESCRIPTION OF THE DRAWING FIGURES

In a Passive Optical Network (PON), each Optical Line Termination (OLT)is connected over an optical fiber plant to several Optical NetworkUnits (ONUs). The OLT is a unit typically located at a service providerpremise and the each ONU is located at or near a subscriber premise. TheONUs take turns transmitting data to the OLT. Due to differences inoutput calibration, fiber loss, etc. an OLT can see a relatively strongoptical signal from one ONU and a relatively weak signal from anotherONU. This variance in received optical signal strength can make itdifficult (and expensive) for an OLT to correctly recover input data.

To reduce adverse effects associated with such variations in signalstrength, it is advantageous to implement a power levelling mechanismfor reducing the range of input optical power seen at the receiver ofthe OLT. The power levelling mechanism sends messages to the ONUs thatcause them to adjust their output power. However, the OLT must know withsome degree of accuracy what power level it receives from each ONU inorder to make the decision as to how the ONU output power should beadjusted. Current OLT receiver technology does not provide thismeasurement as a standard feature, therefore requiring separate receivedsignal strength indication (RSSI) circuitry to be implemented. OLT's inaccordance with the present invention include such RSSI circuitry and,thereby, support such power levelling functionality.

In addition to the requirements for power levelling, it is desirablethat an OLT be able to provide input power values in units of dBm on aper-ONU basis for the purpose of non-intrusive monitoring of systemconditions by a system operator (e.g., systems performance monitoring orperiodic maintenance). Accordingly, optical input power measurementsfacilitated in accordance with the present invention may be used to meetthe power levelling recommendation and can serve this purpose as well.

Gigabit PON (GPON), which is defined in ITU-T Recommendation G.984, is aspecific example of a PON technology into which an OLT in accordancewith the present invention may be implemented for the purpose ofenabling power levelling functionality. However, it is disclosed hereinthat power levelling and the corresponding benefit of RSSI circuitry inaccordance with the present invention will be applicable to otheroptical network technologies and, perhaps, other applications ingeneral.

The GPON Physical Medium Dependent (PMD) layer specification (i.e., inITU-T Recommendation G.984.2, 03/2003) describes a power levellingmechanism that can be implemented on an OLT in order to relax dynamicrange requirement for the OLT burst-mode receiver. The specificationincludes a number of specific recommendations for such a mechanism. Afirst specific recommendation is that the OLT measures the averageoptical input power (P) of each ONU burst. This constrains the responsetime of the measurement technique because the measurement must beperformed during normal operation of the PON. The OLT controls the bursttime allocated for each ONU so the ONU burst time can vary. Typicalburst times are expected to be on the order of microseconds. Largerburst times can be allocated by the OLT, but will tend to add latency totraffic from other ONUs. A second specific recommendation is that theOLT Rx must be able to measure the burst power at 5 dB below sensitivity(as defined by ITU G.984). This recommendation serves to define thedynamic range of the measurement technique. The Rx sensitivityrequirement is on the order of −30 dBm (dB milliwatts) and the upper endof the input power range could approach −5 dBm. This results in adynamic range of 30 dB for optical input power or 60 dB for theelectrical current output of a photodiode of the OLT. A third specificrecommendation is that the uncertainty range of the measurement over thefull operating range of the OLT is 4 dB maximum. This defines the errortolerance of the measurement over the operating range of the OLT,including variations in temperature and input power level. Powermeasurement functionality in accordance with the present inventionprovides a means for accommodating these recommendations.

In addition to the requirements for power levelling, it is desirablethat the GPON OLT be able to provide input power values in units of dBmon a per-ONU basis for the purpose of non-intrusive monitoring of systemconditions by a system operator (Systems Performance Monitoring or PM).The measurements used to meet the power levelling recommendation servethis purpose.

FIG. 1 depicts an embodiment of an optical line termination inaccordance with the present invention, which is referred to herein asthe optical line termination (OLT) 100. The optical line termination 100comprises a photodiode DC power supply 105, received signal strengthindication (RSSI) circuitry 110 and a photodiode 115. The RSSI circuitry110 is connected between the photodiode DC power supply 105 and thephotodiode 115. The DC power supply 105 provides input power current tothe photodiode 115 during signalling operation of the photodiode 115.The photodiode DC power supply 105 and the photodiode 115 areconventional OLT components and, thus, will not be discussed insignificantly greater detail herein.

The RSSI circuitry 110 includes a DC current load 120, power measurementcircuitry 125 and current mirror circuitry 130. The DC current load 120is connected in parallel with the photodiode 115, which contributes toreducing overall response time of the RSSI circuitry 110. The currentmirror circuitry 130 is connected between the DC current load 120, theDC power supply 105 and the power measurement circuitry 125. As withcurrent flow in conventional photodiode OLT implementations, an opticalinput signal results in the photodiode DC power supply 105 providingelectrical current to the photodiode 115 (i.e., photodiode currentI_(p)). However, in accordance with the present invention, parallelconnection of the photodiode 115 with the DC current load 120 results inthe photodiode DC power supply 105 providing electrical current to theDC current load 120 (i.e., DC load current I_(L)). Together, thephotodiode current I_(P) and the DC load current I_(L) represent anaggregate current (i.e., aggregate current I_(A)). Preferably, the DCcurrent load 120 implements temperature compensation techniques toprovide an offset current that varies little with temperature.

The current mirror circuitry 130 provides a copy of the aggregatecurrent I_(A) to the power measurement circuitry 125 (i.e., mirrorcurrent I_(M)), which enables measurement of the photodiode currentI_(P) to be performed without regard to the data path implementation.Advantageously, measurement of the photodiode current I_(P) inaccordance with the present invention is independent of data pathcomponents (and vice-versa) so that improvements to either the RSSIcircuit or data path circuit can be implemented without affecting oneanother.

Preferably, the DC current load 120 exhibits a substantially fixed loadand, accordingly, draws a known current (i.e., the DC load currentI_(L)) such that the mirror current I_(M) has a known relationship tothe photodiode current I_(P) and can be measured without affectingreceiver circuitry of the OLT. The photodiode DC power supply 105, thephotodiode 115 and other conventional circuitry of the OLT 100 thatreceives ONU signals and transmits them upstream are examples of thereceiver circuitry. The current mirror 130 advantageously serves toisolate receiver circuit of the OLT 100 from the power measurementcircuitry 125, which is advantageous because location of the photodiodecircuit 115 in the OLT 100 is very sensitive to noise contamination.

The power measurement circuitry 125 includes a first analog filter 131,a logarithmic amplifier 135, a second analog filter 140, ananalog-to-digital converter 145, a conversion initiation module 150 anda power computation module 155. The logarithmic amplifier 135 isconnected between the first analog filter 131 and the second analogfilter 140. The second analog filter 140 is connected between thelogarithmic amplifier 135 and the analog-to-digital converter 145. Theconversion initiation module 150 and a power computation module 155 areeach connected to the analog-to-digital converter 145.

As disclosed above, the current mirror circuitry 130 provides a copy ofthe aggregate current I_(A) to the power measurement circuitry 125(i.e., mirror current I_(M)). The mirror current I_(M) is in an analogform. Preferably, but not necessarily, the mirror current I_(M) includesgain (i.e., I_(M)=Gain*I_(P)+I_(L)). The mirror current I_(M) isprovided from the current mirror circuitry 130 (i.e., via a mirrorcurrent output) to an input of the logarithmic amplifier 135 through thefirst analog filter 131. The first analog filter 131 provides forfrequency compensation and noise rejection to the mirror current I_(M).The logarithmic amplifier 135 outputs an analog voltage dependent uponan average of the mirror current I_(M) (i.e., converts the mirrorcurrent I_(M)). The average of the mirror current I_(M) is converted tothe analog voltage on a logarithmic scale, which increases dynamic rangeand simplifies calculation of photodiode input power in units of dBm.For example, a 1 μA mirror current I_(M) corresponds to a 0.2V analogvoltage while a 10 μA mirror current I_(M) corresponds to a 0.4V analogvoltage, a 100 μA mirror current I_(M) corresponds to a 0.6V analogvoltage, etc. Advantageously, the conversion of the mirror current I_(M)on a logarithmic scale improves dynamic range by converting a wide rangeof input power levels to a voltage range that is suitable formeasurement by a relatively low-cost analog-to-digital converter in atypical central office environment (i.e. in the presence of noise fromlocal switching power supplies and other electronics).

The logarithmic amplifier 135 largely influences the response time ofthe RSSI circuitry 110. The response time of known integratedlogarithmic amplifiers (e.g., the logarithmic amplifier 135) varies inproportion to an applied input current (i.e., the mirror current I_(M)).When initial input power (i.e., derived from the mirror current I_(M))is very low, the response time for the logarithmic amplifier 135 can beunacceptably large. For example, response time for a typical logarithmicamplifier is several hundred microseconds when starting at zero inputpower and going to a very low average input power. Response timesgreater than about 100 microseconds can place unacceptable constraintson upstream ONU bandwidth allocation. Accordingly, the DC current load120 is advantageously connected in parallel with the photodiode 115(i.e., a shunt load) to improve overall response time. Inclusion of theDC current load 120 serves to sink a constant amount of current so thatthe input of the logarithmic amplifier 135 exhibits input currentoffset. The sink current generated by the DC current load 120 issufficiently large to move the logarithmic amplifier 135 into anoperating region where the response time for any valid input isacceptable. For example, a sink current of 1 microamp may result in amaximum response time of up to about 50 microseconds. The sink currenthas the effect of raising the offset voltage at the logarithmicamplifier 135 output, but does not affect its logarithmic gain.

The analog voltage outputted by the logarithmic amplifier 135 isprovided to an input of the analog-to-digital converter 145 (i.e., ananalog-to-digital converter input) through the second analog filter 140.The second analog filter 140 provides for frequency compensation andnoise rejection to the analog voltage. The analog-to-digital converter145 outputs a digital voltage dependent upon the analog voltage (i.e.,converts the analog voltage).

The analog voltage outputted from the logarithmic amplifier 135 iscontinually provided to the analog-to-digital converter 145. Inaccordance with the present invention, the conversion initiation module150 initiates analog-to-digital conversions for facilitating a powercomputation corresponding to a specific photodiode burst. It isdisclosed herein that the power computation for a specific photodiodeburst power computation may be triggered either local to the OLT or froma higher level of the system. For example, the conversion initiationmodule 150 and the power computation module 155 may be components of aGPON MAC interface through which power computation may be triggered. Inthis example, the MAC Interface allocates an upstream timeslot for aparticular optical network unit (ONU) to be measured and knows (e.g.,per standard GPON operation) when the ONU burst starts. MAC control ofthe analog-to-digital conversions contributes to tight coordination ofphotodiode input power measurements with respect to ONU bursts. When theupstream burst starts (i.e., is energized), the MAC interface starts acounter of the conversion initiation module 150 whose terminal value hasbeen set based on the system configuration. Preferably, the terminalvalue represents the settling time of the RSSI circuitry 110 plus somemargin. When the counter reaches its terminal value, the conversioninitiation module 150 triggers an analog-to-digital conversion of theanalog voltage. In this manner, response time of the RSSI circuitry 110is consistent with a single ONU burst so that input power measurementscan be made during normal operation of the photodiode 115 with reducedimpact on signalling traffic.

When the analog-to-digital conversion is complete, the analog-to-digitalconverter 145 triggers the power computation module for facilitatingcomputation of photodiode input power in dBm dependent upon the digitalvoltage outputted by the analog-to-digital converter. In one embodimentof the present invention, the conversion of digital voltage tophotodiode input power (P_(in)) in dBm is defined by a calculation ofthe form:P _(in)(dBm)=A×V _(out) +B+f(V _(out)),

where A and B are constants and f(V_(out)) is a function of V_(out)which accounts for the offset current from the photodiode and DC currentsink when there is zero optical input. Preferably, the values of A, Band f(V_(out)) are calculated and stored when the OLT 100 is calibratedso that only arithmetic operations (e.g., multiplication and additionoperations) need be performed at run-time.

In an alternate embodiment of the present invention, a resistor or otherlinear conversion circuit is used instead of a logarithmic amplifier toconvert current to voltage. In this case, the DC current load is notrequired to improve the overall response time. However, the DC currentload still provides benefits associated with moving theanalog-to-digital converter input away from 0V at low input powerlevels. Such an alternate embodiment may potentially exhibit some of thesame issues as the known existing input power measurement solutions withrespect to dynamic range and accuracy. Advantageously, however, thisalternate embodiment of the present invention provides a means ofmeasuring optical input power for each ONU during normal operation thatis independent of the data path components and that has minimal impacton signalling traffic.

Turning now to a discussion of a specific utility for input powermeasurement functionality in accordance with the present invention, theburst-mode receiver circuitry on a typical OLT in a PON is typicallydesigned to support a specific optical power range (e.g. −27 dBm to −6dBm), which is based on network parameters such as bit rate and OpticalDistribution Network (ODN) class. Existing PON systems use open-looptechniques to set the receiver operating point with respect to measuredinput power from each ONU and only account for conditions at the OLT(such as temperature) and not conditions on the PON. However, in asystem in accordance with the present invention where the input powerfrom each Optical Network Unit (ONU) can be measured and the inputoptical-to-electrical gain can be adjusted, the input range of the OLTcan be inventively controlled in closed-loop fashion to account fordifferent optical network level conditions. Advantageously, thiscapability can extend the operating range by as much as an additional 6dBm (e.g. −30 dBm to −3 dBm).

PON technologies such as GPON (i.e., defined in ITU RecommendationG.984) require that the OLT receiver circuitry support a wide dynamicrange of burst-mode optical input power levels from multiple OpticalNetwork Units (ONU). This is typically accomplished by optimizing thereceiver operating point at some initial condition such that systemrequirements are met over operating conditions such as temperature andaging. As discussed above, the GPON standard describes a power levellingmechanism enabling ONU output power levels to be grouped. Thus, powerlevelling can be used to reduce the dynamic range requirement of the OLTreceiver by about 6 dB. But, undesirably, conventional approaches forfacilitating ONU power adjustments are coarse and limited in range.

Where the OLT receiver uses a variable gain photodiode such as, forexample, an Avalanche-type Photodiode (APD) then the receiver opticalinput range can be adjusted by controlling the APD voltage bias becausethe APD optical-to-electrical gain depends on the bias level. In somesituations, it will be desirable to adjust the operating range of theOLT receiver during operation in order to account for optical linkconditions not supported by the original receiver operating point. Oneexample of such a situation is a system where the input power from oneor more ONUs is higher than the supported range and therefore violatesthe receiver input overload level. In this case, the receiver operatingrange may be shifted upward (by decreasing APD gain) to support a higherinput overload level. Another example of such a situation is a systemwhere the input power from each ONU has decreased significantly overtime, indicating a change in the ODN or receiver characteristics. Inthis case, the receiver operating range may be shifted downward (byincreasing APD gain) to improve bit error ratio (BER) at low inputlevels.

To this end, facilitating photodiode input power measurement (i.e., ONUinput power measurement) with an RSSI circuitry in accordance with thepresent invention enables accurate measurement of the input power fromeach ONU on a PON. Accurate measurement of input power allows the OLT todetermine conditions such as whether an ONU is operating outside anallowed range or how much the power from a group of ONUs has changedover time. Thus, implementation of RSSI circuitry in accordance with thepresent invention in combination with suitable photodiode gain controlprovides for a closed-loop system that can adjust photodiode gain inresponse to certain PON conditions.

An advantage of such a combined implementation of such RSSI circuitryand photodiode gain control is that the OLT receiver gain is adjusted toaccount for measured changes in PON conditions and receivercharacteristics. This enables the OLT to be configured for takingcorrective action and extending the input operating range of the OLT.Additionally, these adjustments can be made with relatively fineresolution and with relatively wide range.

The voltage bias of the APD used on the OLT is controlled using acircuit that sets the operating point for a DC-DC power supply.Increasing the voltage bias has the effect of increasing theoptical-to-electrical gain of the APD. Typically, an OLT uses an APDbias that provides maximum Signal-to-Noise Ratio (SNR) and only adjuststhe bias to account for changes in temperature since the bias point formaximum SNR will change with temperature.

Referring now to FIG. 2, an embodiment of a GPON OLT configured forproviding closed-loop photodiode gain control in accordance with thepresent invention is depicted. The GPON OLT is referred to herein as theOLT 200. The OLT 200 includes a photodiode DC power supply 205, a RSSIcircuit 210, a photodiode 215, GPON receiver datapath circuitry 220 andGPON MAC circuitry 225. The RSSI circuitry 210 is substantially the sameas that depicted in FIG. 1 and is connected between the DC power supply205, the photodiode 215 and the GPON MAC circuitry 225. Preferably, thephotodiode 215 is an Avalanche-type photodiode or, optionally, aphotodiode that offers suitable bias-level dependent gain.

The photodiode 215 is connected between the RSSI circuitry 210 and theGPON receiver datapath circuitry 220. A closed-loop path 230 is providedbetween the DC power supply 205, the RSSI circuit 210 and the GPON MAC225. Through control of the Photodiode DC power supply 205, results ofRSSI input power measurement as discussed above in reference to FIG. 1are used to set a particular output of the Photodiode DC power supply205 (e.g., via a control portion of the Photodiode DC power supply 205).

FIG. 3 depicts an embodiment of a control loop used in the OLT 200 inFIG. 2, which is referred to as the control loop 300. An output of aphotodiode optical-to-electrical transfer function 305 is connected inseries with an input of a RSSI circuitry transfer function 310. Measuredoptical input power levels P(n) for all ONU's of the OLT 200 areprovided at an output of the RSSI circuitry transfer function 310. Themeasured optical input power levels P(n) are provided to an input of anphotodiode bias adjustment logic 315. A photodiode bias adjustmentparameter B(adj) is provided at an output of the photodiode biasadjustment logic 315. The photodiode bias adjustment parameter B(adj) iscompared to a reference voltage V(ref) with the resulting voltagedifference being applied at an input to the photodiodeoptical-to-electrical transfer function 305.

In the preceding detailed description, reference has been made to theaccompanying drawings that form a part hereof, and in which are shown byway of illustration specific embodiments in which the present inventionmay be practiced. These embodiments, and certain variants thereof, havebeen described in sufficient detail to enable those skilled in the artto practice embodiments of the present invention. It is to be understoodthat other suitable embodiments may be utilized and that logical,mechanical, chemical and electrical changes may be made withoutdeparting from the spirit or scope of such inventive disclosures. Toavoid unnecessary detail, the description omits certain informationknown to those skilled in the art. The preceding detailed descriptionis, therefore, not intended to be limited to the specific forms setforth herein, but on the contrary, it is intended to cover suchalternatives, modifications, and equivalents, as can be reasonablyincluded within the spirit and scope of the appended claims.

1. An optical line termination, comprising: a DC current load; powermeasurement circuitry; upstream data path circuitry; and current mirrorcircuitry connected between said DC current load, said power measurementcircuitry and said upstream data path circuitry.
 2. The optical linetermination of claim 1 wherein the DC current load is connected inparallel with a photodiode of said upstream data path circuitry.
 3. Theoptical line termination of claim 2 wherein the DC current load exhibitsa substantially fixed load.
 4. The optical line termination of claim 2wherein: the current mirror provides a copy of an aggregate current tosaid power measurement circuitry; and the aggregate current is asummation of a current draw by the DC current load and a current draw bythe photodiode.
 5. The optical line termination of claim 1 wherein: saidpower measurement circuitry includes analog amplification circuitry andanalog-to-digital converting circuitry; said analog amplificationcircuitry is connected between said current mirror circuitry and saidanalog-to-digital converting circuitry; said current mirror circuitryprovides a copy of an aggregate current to said analog amplificationcircuitry; and the aggregate current is a summation of a current draw bythe DC current load and a current draw by a photodiode of said upstreamdata path circuitry.
 6. The optical line termination of claim 5 whereinsaid power measurement circuitry includes: a first analog filterconnected between said current mirror circuitry and said analogamplification circuitry; and a second analog filter connected betweensaid analog amplification circuitry and said analog-to-digitalconverting circuitry.
 7. The optical line termination of claim 5wherein: said analog amplification circuitry outputs an analog voltagedependent upon the aggregate current; said analog-to-digital convertingcircuitry outputs a digital voltage dependent upon the analog voltage;and said power measurement circuitry includes a power measurement modulethat outputs a power level dependent upon the digital voltage.
 8. Theoptical input power monitoring apparatus of claim 7 wherein: said powermeasurement circuitry includes a counter; said power measurementcircuitry is configured for starting the counter in response to anexpected change in at least one of the analog current and the analogvoltage and for initiating an analog-to-digital conversion of the analogvoltage when the counter attains a prescribed number of counts.
 9. Anoptical input power monitoring apparatus, comprising: current mirrorcircuitry including a first output and a second output, where saidcurrent mirror circuitry is configured for producing a copy of a firstcurrent, for outputting the copy of the first current through the secondoutput and for outputting the first current through the first output;power measurement circuitry connected to the second output.
 10. Theoptical input power monitoring apparatus of claim 9, further comprising:a DC current load connected to the first output.
 11. The optical inputpower monitoring apparatus of claim 9 wherein: said power measurementcircuitry includes analog amplification circuitry and analog-to-digitalconverting circuitry; and said analog amplification circuitry isconnected between said current mirror circuitry and saidanalog-to-digital converting circuitry.
 12. The optical input powermonitoring apparatus of claim 11 wherein said power measurementcircuitry includes: a first analog filter connected between said currentmirror circuitry and said analog amplification circuitry; and a secondanalog filter connected between said analog amplification circuitry andsaid analog-to-digital converting circuitry.
 13. The optical input powermonitoring apparatus of claim 11 wherein: said analog amplificationcircuitry outputs an analog voltage dependent upon the first current;said analog-to-digital converting circuitry outputs a digital voltagedependent upon the analog voltage; and said power measurement circuitryincludes a power measurement module that outputs a power level dependentupon the digital voltage.
 14. The optical input power monitoringapparatus of claim 13, further comprising: a DC current load connectedto the first output.
 15. A method for generating a received signalstrength indication, comprising: providing a copy of an aggregatecurrent, wherein the aggregate current is a summation of a current drawby a DC current load and a current draw by a photodiode of upstream datapath circuitry in a passive optical network; converting the aggregatecurrent to a digital voltage; converting the digital voltage to a powerlevel.
 16. The method of claim 15 wherein providing the copy of theaggregate current includes providing to current mirror circuitry boththe current draw by a DC current load and the current draw by aphotodiode of upstream data path circuitry.
 17. The method of claim 15wherein: converting the aggregate current to the digital voltageincludes providing the aggregate current to analog amplificationcircuitry for generating an analog voltage dependent upon the aggregatecurrent and providing the analog voltage to analog-to-digital convertingcircuitry for generating a digital voltage dependent upon the analogvoltage; and converting the digital voltage to the power level includesproviding the digital voltage to a power measurement module forgenerating a power level dependent upon the digital voltage.
 18. Themethod of claim 17 wherein said converting the analog voltage to thedigital voltage includes: starting a counter in response to energizingthe photodiode; and initiating an analog-to-digital conversion of theanalog voltage when the counter attains a prescribed number of counts.19. The method of claim 18 wherein said converting the digital voltageto the power level is performed in response to completion of saidconverting the analog voltage to the digital voltage.
 20. The method ofclaim 19 wherein providing the copy of the aggregate current includesproviding to current mirror circuitry both the current draw by a DCcurrent load and the current draw by a photodiode of upstream data pathcircuitry.